1. Field of the Invention
The present invention relates to a printed circuit board used for various electronic devices.
2. Description of the Background Art
In a printed circuit board, conductive patterns are on one surface of a base insulating layer and a reinforcement layer is formed on the opposite surface of the base insulating layer in some cases (see JP 2004-134442 A, for example). This reinforces the printed circuit board.
Hereinafter, the printed circuit board on which a semiconductor device is to be mounted will be described briefly.
FIG. 4 is a plan view of a conventional printed circuit board 200. As shown in FIG. 4, a plurality of conductive patterns 2 are formed on abase insulator layer 1 composed of, for example, polyimide. The conductive patterns 2 have those formed to extend from the center of the base insulating layer 1 toward one side of the base insulating layer 1 and the others formed to extend from the center toward the other side of the base insulating layer 1.
A cover insulating layer 3 is provided to cover an area excluding the area of one side and the area of other side of the base insulating layer 1. The area in which the end of each conductive pattern 2 is not covered by this cover insulating layer 3 is referred to as an outer lead portion 20.
In addition, the end of each conductive pattern 2 in the central part of the base insulating layer 1 is mounted with a semiconductor device that is not illustrated. The mounting area of this semiconductor device is shown as a semiconductor device mounting part 10 in FIG. 4. The area where each conductive pattern 2 is positioned within this semiconductor device mounting part 10 is referred to as an inner lead portion 30.
Hereinafter, the part of each conductive pattern 2 on the outer lead portion 20 in the printed circuit board 200 will be referred to as an outer lead wiring 2a. 
Next, how the outer lead wiring 2a is connected to a terminal part of a glass substrate of a liquid crystal display or the like will be described.
FIG. 5 is a perspective view showing how the printed circuit board 200 is connected to the glass substrate of the liquid crystal display.
As shown in FIG. 5(a), a plurality of conductive patterns 2 are provided on the base insulating layer 1. Since a part of the above-mentioned printed circuit board 200 of FIG. 4 is illustrated simplistically in FIG. 5(a), and its configuration is the same as that of the printed circuit board 200 in FIG. 4, the description will not be given.
As illustrated in FIG. 5(b), a plurality of terminal parts (drawn in FIG. 6 which will be described below) are provided on one surface of a glass substrate 4, and an integrated anisotropic conductive film (ACF) 5 is provided on these terminal parts.
As shown in FIG. 5(c), the outer lead wirings 2a of the printed circuit board 200 are connected with the terminal parts on the glass substrate 4 such that the anisotropic conductive film 5 of this glass substrate 4 is jointed onto the outer lead wirings 2a of the printed circuit board 200.
In this case, after the terminal parts on the glass substrate 4 are overlaid on the outer lead wirings 2a such that these terminal parts are attached onto the outer lead wirings 2a by the anisotropic conductive film 5, heat treatment (heat press) is carried out in the temperature environment of about 180° C., for several to several tens of seconds. This allows the terminal parts on the glass substrate 4 to be connected with the outer lead wirings 2a of the printed circuit board 200.
However, the following problems will arise. That is to say, the above-mentioned heat treatment causes expansion of the base insulating layer 1 of the printed circuit board 200 and the glass substrate 4. In this case, the heat treatment for several to several tens of seconds could not necessarily stabilize the coefficient of expansion of the base insulating layer 1 and glass substrate 4.
In addition, the base insulating layer 1 composed of polyimide absorbs moisture at room temperature. The base insulating layer 1 releases the moisture and contracts when the above-mentioned heat treatment is carried out. In this case, even the contraction of the base insulating layer 1 could not be necessarily stabilized due to environmental changes. This causes, as illustrated in FIG. 6(a), misalignment of connection between the respective outer lead wirings 2a of the printed circuit board 200 and each terminal part 6 of the glass substrate 4.
Therefore, in order to reduce the above-mentioned expansion and contraction of the base insulating layer 1, as shown in FIG. 6(b), a back surface pattern (reinforcement pattern) 7a made of metal is attached onto the opposite surface of the base insulating layer 1 to the surface on which the outer lead wirings 2a are placed (see JP 2003-68804 A, for example).
However, when the environmental temperature becomes room temperature after the above-mentioned heat treatment, contraction stress remains in the above-mentioned back surface pattern 7a. In particular, the remaining contraction stress in the area of the outer lead wirings 2a on both ends of the base insulating layer 1 is remarkably large. As a result, the respective connected outer lead wirings 2a and terminal parts 6 might be stripped in some cases.